The present invention relates to a fish finder capable of detecting and discriminating only fish of a predetermined size.
In the past low-frequency fish finders have been used for detecting fish of relatively large size while high-frequency fish finders have been used for detecting fish of relatively small size such as whitebaits, shrimps and the like. However, the high-frequency fish finders can detect all of fish which the low-frequency fish finders can detect so that when the high-frequency fish finders are used for detecting small fish such as whitebaits or shrimps they also detect large fish such as sardines and mackerels and consequently cannot distinguish between large and small fish.
To overcome this problem, the same inventor devised a fish finder capable of detecting and distinguishing small fish such as shrimps, planktons, whitebaits and the like. This fish finder is disclosed in Japanese patent publication No. 40-24444. This fish finder includes two transducers capable of generating high- and low-frequency ultrasonic sound waves in response to the pulses from an oscillator and emitting them simultaneously. The high- and low-frequency echoes reflected back from the targets are converted into the electrical high- and low-frequency signals which are amplified and applied to an extractor circuit, where the signal components common in both the high- and low-frequency signals such as those representative of the bottom and the surface of the sea and individual and schools of fish of large size are extracted or eliminated from the high-frequency signal, whereby only small fish may be detected and distinguished.
The fish finder of the type described must emit the high- and low-frequency ultrasonic sound waves at the same beam angle. However when a school of small fish is dense, the low-frequency ultrasonic sound waves reflected by this school have a relatively high level. Therefore when the high-frequency signal is extracted or eliminated by the low-frequency signal, the signal component representative of this relatively dense school of small fish is extracted or eliminated so that small fish cannot be detected. Furthermore two transducers must be provided for generating high- and low-frequency ultrasonic sound waves, and they are large in size because they must emit the high- and low-frequency ultrasonic sound waves at the same beam angle.